Method for sequestration of carbon dioxide using a body of water and a suspended platform therefor

ABSTRACT

A platform for sequestering carbon dioxide using a body of water is described. The platform has a vessel for holding solid metal hydroxide and for exposing the solid metal hydroxide to a flow of water to create a solution of a metal hydroxide having a pH level. The solution containing metal hydroxide is released into the body of water, causing a reaction with the carbon dioxide present in the body of water, thereby producing metal carbonate/bicarbonate, thus sequestering the carbon dioxide. A choice of the metal in the metal hydroxide, a rate of the releasing the solution containing the metal hydroxide into the body of water, and a flow rate of the flow of water so that to substantially maintain the solution containing the metal hydroxide at the pH level that is defined as environmentally safe and not changing chemistry of seawater. A corresponding method is also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims benefit from U.S. provisional applicationSer. No. 63/210,793 filed on Jun. 15, 2021, the entire contents of whichare incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of carbon capture andsequestration, and in particular to the method for sequestration ofcarbon dioxide using a body of water, and a suspended platform therefor.

BACKGROUND OF THE INVENTION

It is of interest to stabilize if not reduce atmospheric CO₂concentrations in order to avoid deleterious climate and ocean chemistryimpacts. Methods of achieving this include those that remove CO₂ fromwaste gas streams or from air and that then sequester the carbon fromthe atmosphere.

The greenhouse gases (GHG) in the atmosphere are capable of absorbinginfrared radiation, thus, they trap and keep heat in the atmosphere. Byincreasing the heat in the atmosphere, greenhouse gases cause thegreenhouse effect that results in global warming. Among GHGs, carbondioxide (CO₂) is considered as the primary GHG that is mainly emitted asa result of human activities. According to the report by the UnitedStates Environmental Protection Agency, in 2018, 81.3 percent of allU.S. emissions of GHGs was due to CO₂ emissions from human activities.Among all human activities that result in CO₂ emissions, fossil fuelcombustions are one of the main sources of CO₂ emission. To overcomethis issue research on CO₂ reduction (to decrease the amount of GHG inthe atmosphere) has gained attention. Various thermo-chemical andelectrochemical processes have been developed to reduce point-source CO₂emissions as well as to directly remove CO₂ from air.

Among these processes, CO₂ capture through the reaction with certainCO₂-reactive, alkaline chemicals has been explored with CO₂ coming froma variety of concentrated and dilute sources. The addition of alkalinityto surface ocean waters can effect atmospheric CO₂ removal and storagethrough the transformation of seawater-dissolved CO₂ into stablecompounds has also been explored. Through gas equilibrium processes thisremoval of CO₂ from surface seawater causes the removal of CO₂ from theatmosphere, which is equivalent to capturing of CO₂ from the atmosphere.There is significant global potential for such approaches to contributeto atmospheric CO₂ management, assuming such alkalinity addition to theocean can be safely and cost-effectively conducted.

Therefore, there is a need for developing improved or alternativemethods and systems for carbon capture and sequestration, includingsequestration of carbon dioxide using a body of water.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a method forsequestration of carbon dioxide using a body of water, and a floating orsuspended platform therefor.

The invention pertains to methods of introduction of metal hydroxideions into a solution to increase CO₂ uptake and storage by the solution,where the metal hydroxide ions are derived from a solid metal hydroxide.Such solutions may be natural or artificial water bodies that include,but are not limited to ponds, lakes, reservoirs, rivers, and the ocean.

It is of interest to control the release of metal hydroxide ions into awater body so as to stay below concentrations that would cause chemicaland biological harm such as unnecessary precipitation of solid carbonatein seawater. It is also of interest to arrange for the release of metalhydroxide ions into the body of water in a cost effective manner.

Embodiment of the present invention describe the following features:

-   -   1) Forming a solid metal hydroxide and controlling the rate of        its dissolution or release in a body of water by adjusting the        amount of water contacted by the solid metal hydroxide, or        adjusting the surface area of the solid metal hydroxide exposed        to the water.        -   Such adjustments include controlling:            -   i) the flowrate/turbulence of water over the surface of                the metal hydroxide, and            -   ii) the surface area of the solid metal hydroxide                exposed to a given volume of water.    -   2) Contacting solid metal hydroxide with an enclosed volume of        water to produce dissolved metal hydroxide or small particles of        metal hydroxide, and then controlling the release rate of these        ions/particles to the body of water, for example ocean. Such        controls include:        -   i) Metering of the flow/release of the solution containing            the ions/particles into the body of water;        -   ii) Using membranes or other semi-permeable barriers that            separate the enclosed solid metal hydroxide and contacting            volume of water from the external body of water, and whose            permeability allows metal hydroxide ions and possibly very            small metal hydroxide particles to pass at a given rate. By            selecting a membrane/barrier and/or by adjusting the surface            area of the membrane/barrier relative to the volume of water            into which the ions/particles are to be released, the impact            to water chemistry and biology can be controlled; and

According to one aspect of the invention, there is provided a floatingplatform for sequestering carbon dioxide using a body of water,comprising:

(a) a vessel for holding solid metal hydroxide;

(b) means for exposing the solid metal hydroxide to a flow of water tocreate a solution of a metal hydroxide having a pH level;

(b) means for releasing the solution containing the metal hydroxide intothe body of water, including causing a reaction of the released metalhydroxide with the carbon dioxide present in the body of water, therebyproducing one or more of a metal carbonate and metal bicarbonate below asaturation concentration in the body of water, thus sequestering thecarbon dioxide using a body of water; and

(c) means for choosing a rate of the releasing the solution containingthe metal hydroxide into the body of water, and a flow rate of the flowof water so that to substantially maintain the solution containing themetal hydroxide at the pH level that is defined as environmentally safefor the body of water.

In the floating platform described above, the metal hydroxide ismagnesium hydroxide, The rate of the releasing the solution is chosen ofabout 3 mmoles dissolved Mg(OH)₂/(L×m²×day).

In the embodiments of the invention, the desired maximum pH level is 9.0and less desireably from about 9.0 to about 9.4, and preferably notexceeding 9.4.

According to another aspect of the invention, there is provided a methodfor sequestering carbon dioxide using a body of water, comprising:

(a) in a vessel containing solid metal hydroxide, exposing the solidmetal hydroxide to a flow of water to create a solution containing themetal hydroxide and having a pH level;

(b) releasing the solution containing the metal hydroxide into the bodyof water, including causing a reaction of the released metal hydroxidewith the carbon dioxide present in the body of water, thereby producingone or more of a metal carbonate and metal bicarbonate at belowsaturation concentration in the body of water, thus sequestering thecarbon dioxide using a body of water;

(c) choosing a rate of the releasing the solution containing the metalhydroxide into the body of water in the step (b) and a flow rate of theflow of water in the step (a) so that to substantially maintain thesolution containing the metal hydroxide at the pH level that is definedas environmentally safe for the body of water.

In the method described above, the metal hydroxide is magnesiumhydroxide. The step (c) comprises choosing the rate of the releasingequal to about 3 mmoles dissolved Mg(OH)₂/(L×m²×day).

In the method described above, the pH level is from about 9.0 to about9.4. In the embodiments of the present invention, the pH level is notexceeding 9.4.

Thus, an improved method for sequestration of carbon dioxide using abody of water and a corresponding platform have been provided.

BRIEF DESCRIPTION OF DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent application publication with colordrawing(s) will be provided by the Office upon request and payment ofthe necessary fee.

Embodiments of the present invention will be further described withreference to the accompanying exemplary drawings, in which:

FIG. 1 illustrates an initial increase of seawater pH values versus timeupon adding different additions of magnesium hydroxide to 1 L ofseawater;

FIG. 2A illustrates change of seawater pH values over many days afteraddition of various amount of magnesium hydroxide powder to 1 L ofseawater,

FIG. 2B illustrates the change in dissolved inorganic carbon (DIC) overmany days after addition of various amounts of magnesium hydroxidepowder added to 1 L of seawater;

FIG. 2C illustrates the change of alkalinity (AT) over many days afteraddition of various amount of magnesium hydroxide powder to 1 L ofseawater;

FIG. 3 shows a table summarizing results of elemental analysis of asample of dissolved excess Mg(OH)₂ in seawater;

FIG. 4 shows Energy-Dispersive X-ray Spectroscopy (EDS) analysis ofdissolved Mg(OH)₂ in seawater;

FIG. 5A shows EDS mapping of dissolved Mg(OH)₂ sample in seawater;

FIG. 5B shows EDS mapping of dissolved O in seawater;

FIG. 5C shows EDS mapping of dissolved C in seawater;

FIG. 5D shows EDS mapping of dissolved Mg in seawater;

FIG. 5E shows EDS mapping of dissolved Ca in seawater;

FIG. 5F shows EDS mapping of dissolved Na in seawater;

FIG. 6A schematically illustrates a prospective view of a passivefloating platform of an embodiment of the invention;

FIG. 6B illustrates a side cross section view of a fully loaded passivefloating platform of FIG. 6A floating above sealevel;

FIG. 6C illustrates a side cross section view of the fully loadedpassive floating platform of FIG. 6A floating at about sealevel;

FIG. 6D illustrates a side cross section view of the partially loadedpassive floating platform of FIG. 6A;

FIG. 6E illustrates a side cross section view of the passive floatingplatform of FIG. 6A with all magnesium hydroxide in the hull beingdissolved; and

FIG. 7A illustrates another embodiment of the floating platform withpumping the seawater into the hull and pumping the magnesium hydroxidesolution out of the hull into the ocean;

FIG. 7B illustrates the container 732 of FIG. 7A in more detail;

FIG. 8A illustrates an intermittent mode of operation of the floatingplatform of FIG. 7A;

FIG. 8B illustrates a continuous mode of operation of the floatingplatform of FIG. 7A;

FIG. 9 schematically illustrates the floating platform of FIG. 6A pulledby a ship;

FIG. 10 illustrates the aeration of seawater within the floatingplatform that is aerated to facilitate atmospheric CO₂ removal andsequestration and to facilitate lowering of pH prior to discharge in theocean; and

FIG. 11 illustrates a side view of of another embodiment showing theplacement in a water flow of a mass of Mg(OH)₂ solids encased in asemi-permeable container so as to facilitate Mg(OH)₂ dissolution andproduction of dissolved Mg(OH)₂ prior to the water flowing into theocean.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In our experiments, we have taken into account a relatively lowsolubility of magnesium hydroxide in water (approximately 0.01 g/L, anda solubility product of about 3×10⁻¹¹), and observed that an otherwisepure water solution saturated with magnesium hydroxide can only attain amaximum localized pH of about 10.3.

Additionally, our experimentation has shown that, when 3 mmoles/Lmagnesium hydroxide power was added at seawater, the maximum localizedpH achieved in the seawater reaches about 9.3 after about 2 hr, withmaximum pH values correspondingly declining with lower additions ofMg(OH)₂, as illustrated in FIG. 1 .

In more detail, a diagram 10 of FIG. 1 illustrates an increase oflocalized pH values over time measured in hours, upon adding increasingamounts of magnesium hydroxide powder to seawater, namely:

plot 11 corresponds to an addition of 3 mmol/L Mg(OH)₂ powder;

plot 12 corresponds to an addition of 1 mmol/L Mg(OH)₂ powder;

plot 13 corresponds to an addition of 0.33 mmol/L Mg(OH)₂ powder; and

plot 14 corresponds to control measurements, with no Mg(OH)₂ being addedto the seawater.

This rise in pH reflects the dissolution of the particulate Mg(OH)₂forming dissolved Mg(OH)₂ that elevates solution alkalinity.

Once the localized pH value reaches its maximum, the localized pH valuesdecline over a typical period of several days, reaching correspondinglower pH plateau values afterwards.

FIG. 2A illustrates a decline of localized pH values over time, fordifferent initial additions of magnesium hydroxide powder being added tothe seawater, namely:

plot 21 a corresponding to an addition of 3 mmol/L Mg(OH)₂ powder;

plot 22 a corresponding to an addition of 1 mmol/L Mg(OH)₂ powder;

plot 23 a corresponding to an addition of 0.33 mmol/L Mg(OH)₂ powder;and

plot 24 a corresponds to control measurements, with no Mg(OH)₂ beingadded to seawater.

FIG. 2B illustrates a dependence of dissolved inorganic carbon (DIC)versus time measured in days, for different concentrations of magnesiumhydroxide being added to the seawater, namely:

plot 21 b corresponding to an addition of 3 mmol/L Mg(OH)₂ powder;

plot 22 b corresponding to an addition of 1 mmol/L Mg(OH)₂ powder;

plot 23 b corresponding to an addition of 0.33 mmol/L Mg(OH)₂ powder;and

plot 24 b corresponds to control measurements, with no Mg(OH)₂ beingadded to seawater.

FIG. 2C illustrates a dependence of Alkalinity (AT) versus time measuredin days, for different concentrations of magnesium hydroxide being addedto the seawater, namely:

plot 21 c corresponding to an addition of 3 mmol/L Mg(OH)₂ powder;

plot 22 c corresponding to an addition of 1 mmol/L Mg(OH)₂ powder;

plot 23 c corresponding to an addition of 0.33 mmol/L Mg(OH)₂ powder;and

plot 24 c corresponds to control measurements, with no Mg(OH)₂ beingadded to the seawater.

This dissolved Mg(OH)₂ in turn consumes dissolved CO₂ and converts it tobicarbonate and carbonate ions as follows:

Mg²⁺+2OH—+A(CO_(2aq))--->Mg²⁺+B(HCO₃—)+C(CO₃ ²⁻+H₂O)  (Eq. 1)

where B and C are the molecular fractions of dissolved magnesiumbicarbonate and magnesium carbonate, respectively, where A=B+C. Thepartitioning of the carbon into bicarbonate and carbonate ion isdictated by the solution pH, with the fraction (C) in carbonate ionincreasing with pH above neutral, while the bicarbonate fraction (B)decreases. For example, at a seawater pH of 8, A, B and C areapproximately 1.85, 1.68 and 0.17 respectively, whereas at pH of 9, A, Band C are about 1.35, 0.70 and 0.65, respectively.

In the context of maximizing net CO₂ reaction and removal, maximizing A(minimizing C) is desirable. This means that for maximizing CO₂ removaland storage, it is advantageous to minimize the pH above ambientseawater and/or to minimize duration of pH elevation, for examplerapidly return seawater pH to near ambient values (near pH of about8.1). This decline in pH can be achieved by the dissolution ofatmospheric CO₂ into seawater caused the air-solution CO₂ disequilibriumas a result of the solution's CO₂-absorbing reaction shown inEquation 1. This explains the decline in pH from maximum values as seenin FIG. 2 . When Mg(OH)₂ is locally added to the ocean, the initiallyelevated pH can also decline to near ambient values (for example, 8.1)through dilution of the dissolved Mg(OH)₂ generated with a vastly largerquantity of ambient seawater.

It is also advantageous to lower seawater pH from elevated values sinceseawater CO₃ ²⁻ concentration, and hence degree of saturation of CaCO₃,naturally present in seawater, increases with increasing pH. This canlead to undesirable precipitation of CaCO₃ from seawater via thisreaction:

Ca²⁺+2HCO₃ ⁻--->CaCO_(3s)+CO_(2g)+H₂O  (Equation 2)

Spontaneous precipitation of CaCO₃ and degassing of CO₂ from seawatercan happen at a seawater pH of near 9 and above, in which case thenatural carbon concentration and retention in seawater and seawateralkalinity are undesirably lowered.

In this context, production of solid CaCO₃ was observed in ourpreliminary lab experiments, results of which are illustrated in FIGS.3-5 .

Namely:

FIG. 3 shows a table 100 summarizing results of elemental analysis of asample of dissolved excess Mg(OH)₂ in seawater;

FIG. 4 shows Energy-Dispersive X-ray Spectroscopy (EDS) analysis 200 ofdissolved Mg(OH)₂ in seawater;

FIG. 5A shows EDS element mapping of dissolved Mg(OH)₂ sample inseawater;

FIG. 5B shows EDS element mapping of dissolved O in seawater;

FIG. 5C shows EDS element mapping of dissolved C in seawater;

FIG. 5D shows EDS element mapping of dissolved Mg in seawater;

FIG. 5E shows EDS element mapping of dissolved Ca in seawater;

FIG. 5F shows EDS element mapping of dissolved Na in seawater.

The above noted effect of precipitation of CaCO₃ is seen in the 3 mmol/Lexperiment (FIG. 2 ) wherein the initially high pH level exceeds thepreceding precipitation threshold, causing CaCO₃ precipitation andgeneration of CO₂ (via Equation 1) and thus lowering solution alkalinity(AT) and DIC.

The desired level of pH can thus be achieved by controlling theproduction, release and/or dilution rate of the dissolved metalhydroxide as well as by the degree of in gassing of CO₂ from theatmosphere.

In the treatments with less than 3 mmol/L Mg(OH)₂ added, the deficit indissolved CO₂ (relative to air concentrations) so created by thereaction in Equation 1 then forces air CO₂ to invade the solution,depressing pH (as shown in FIG. 2A, plots 22 a and 23 a) and elevatingthe total dissolved inorganic carbon (DIC) in solution (as shown in FIG.2B, plots 22 b and 23 b). Such DIC increases provide is a direct measureof the desired removal of CO₂ from air into the ocean, and its stablestorage and sequestration in seawater.

As can be seen in the long-term monitoring of the 3 mmoles/L treatment(FIGS. 2B and 2C), the total dissolved inorganic carbon (DIC) and thealkalinity (AT) both abruptly decline after day 1 of the treatment. Thisindicates the undesirable outcome where seawater pH has become elevatedto the point that CaCO₃, naturally present in seawater, becomes sosaturated that it spontaneously precipitates, thus undesirably removingboth DIC and AT from seawater. It is therefore clear that theconcentration of dissolved Mg(OH)₂ released to the ocean needs to becontrolled so as to avoid undesirable precipitation of the CaCO₃naturally present in seawater, to maximize (per mol of Mg(OH)₂ added)the transfer of CO₂ from air into storage as DIC in seawater and to alsoavoid elevations in pH that would exceed legal pH dischargerequirements, for example pH of about 9.0.

These observations indicate that we need to control the release rate ofMg(OH)₂ such that seawater AT concentrations do not exceed about 4mmole/L, which means that if ambient seawater has an ambient AT of 2.2moles/L (FIG. 2 c ), a limit of about 1.8 mmoles/L of additionalalkalinity or about 0.9 mmoles/L of dissolved Mg(OH)₂/L can be added toambient seawater over the course of about 1 day.

This implies that we need to control the dissolution rate of the solidMg(OH)₂ and addition of dissolved Mg(OH)₂ so as not to exceed about 0.9mmoles/(L×day). With surface area/mass of the original powdered Mg(OH)₂of about 5 m²/gram, or about 0.3 m²/mmole, the implied maximum dissolvedMg(OH)₂ release rate becomes about 3 mmoles of Mg(OH)₂/(L×m²×day) or anAT production rate of about 6 mmoles AT/(L×m²×day).

It therefore follows that any combination of i) volume of seawatercontacting and diluting the Mg(OH)₂, ii) surface area of solid Mg(OH)₂contacted by the preceding volume and iii) the duration of the contactthat yields a value at or below the preceding value of the release rateof about 3 mmoles dissolved Mg(OH)₂/(L×m²×day) will satisfy therequirement of staying within desired seawater chemical limits.Dissolution rates below the preceding may be used to further reduce themaximum pH attained, but this will also reduce the quantity of CO₂removal and storage achieved considering the proportionality of Mg(OH)₂addition to CO₂ removal (Equation 2). For example, a dissolved Mg(OH)₂release rate of 1.5 mmoles/(L×m²×day) would reduce in half the amount ofCO₂ removed and stored per unit time relative to the preceding example.

The invention therefore uses the preceding calculation to designdistribution systems for release of dissolved metal hydroxide from solidforms into seawater.

As mentioned above, embodiments of the present invention provide amethod and system for dissolving metal hydroxide into seawater so as tomaintain seawater pH within safe environmental limits while maximizingCO₂-removal and sequestration and avoiding prolonged and/or undesirablechanges to seawater chemistry, including avoiding precipitation of CaCO₃naturally present in seawater.

The embodiments of the invention use the above noted observations andexperiments by manufacturing floating or suspended distribution systemsfor the addition of the magnesium hydroxide to seawater.

One embodiment of the invention describes a passive floatingplatform/vessel 600 schematically illustrated in FIG. 6A, which is madebuoyant, for example through the use of a floating platform 600supported by buoyant sections 612 around the perimeter (for simplicity,only two buoyant sections 612 on two sides of the platform 600 are shownin FIG. 6A). The Mg(OH)₂ 620 is loaded into a sunken opening or hull 630of the floating platform 600 as a solid mass 620 or as a collection ofsolid Mg(OH)₂ masses such as pellets, bricks, or disks, or chunks ofnatural Mg(OH)₂ (brucite mineral) as extracted from a brucite mine.

The buoyant sections 612, the size of the opening 630, and the mass ofMg(OH)₂ to be loaded in the opening 630 are chosen so that an uppersurface 632 of the sunken opening 630 is above the sealevel 634 or atleast floating at the sealevel 634 when the opening 630 is loaded withsolid magnesium hydroxide or forms thereof, and any seawater contactingthe surfaces of the solid magnesium hydroxide, but lower than an averageheight (H) of natural ocean waves in the part of the ocean where thefloating platform 600 is placed. Further the hull 630 may have a sideopening 650 and/or a bottom opening 661 for allowing the seawater toenter the hull 630 and the alkaline water to exit the hull 630, as willbe described in more detail below with regard to FIGS. 6A and 6B.

FIG. 6B shows a side cross section view of the floating platform 600with the opening 630 loaded with solid magnesium hydroxide and anyseawater into which the solid or solids are submerged, when the platform600 floats with the upper surface 632 above the sealevel 634, and thebottom surface 638 b of the opening 630 being at the bottom 638 b level.The bottom surface 638 b may have a bottom hole or bottom opening 661,for discharging the alkaline water out of the hull 630 and also allowingthe seawater to enter the hull 630 from the bottom via natural oceanwaves, the bottom opening 661 optionally may have a permeable membranesimilar to that described below with regard to the side opening 650.

FIG. 6C shows a side cross section view of the floating platform 600with the opening 630 loaded with solid magnesium hydroxide and anyseawater contacting said solid magnesium hydroxide, when the platform600 floats with the upper surface 632 at about the sealevel 634, and thebottom surface of the opening 630 being at the bottom level 638 c whichis deeper than the bottom level 638 b of FIG. 6B with regard to thesealevel 634.

The solid magnesium hydroxide 620 is then bathed in seawater bypassively allowing the opening 630 to partially or completely flood withseawater delivered by ocean waves having a height of H, for example bygetting seawater into the floating platform 600 via ocean waves toppingover the side of the floating platform 600.

As the magnesium hydroxide 620 gets diluted in the seawater, consumesdissolved CO₂ and converts it to bicarbonate and carbonate ions asdescribed above in the Equation (1), the mass of the magnesium hydroxide620 in the opening 630 gradually decreases, while the volume of theseawater partially filling in the opening 630 proportionately increases.Because the specific mass of the seawater is smaller than that ofmagnesium hydroxide, this results in the floating platform 630 graduallyrising above the sealevel 634 as the magnesium hydroxide 620 getsdiluted.

This is illustrated in FIG. 6D and FIG. 6E.

In more detail, FIG. 6D illustrates the opening 630 partially filled inwith the solid magnesium hydroxide 620 and partially filled in withseawater 621, with the bottom surface 638 d of the floating platform 600being at the bottom level 638 d, which is higher than the bottom level638 of FIG. 6B by about Δ1 distance.

Similarly, FIG. 6E illustrates the opening 630 completely filled in withseawater 621 when all solid magnesium hydroxide is diluted, with thebottom surface 638 e of the floating platform 600 being at the bottomlevel 638 e, is higher than the bottom level 638 of FIG. 6B by about Δ₂distance, and Δ₂ is greater than Δ₁.

In the embodiment illustrated by FIGS. 6A, 6B, 6C, 6D and 6E, thedischarge of the alkalized water out of the opening 630 into the oceanis performed by just sloshing out the alkalized water of the top side ofthe opening 630 of the floating platform 600, or additionally oralternatively, by forming the side opening 650 on the side of thefloating platform 600, or forming the bottom opening 661 or in thebottom of the hull 630, as mentioned above.

In order to ensure that all remaining solid magnesium hydroxide in theopening 630 is exposed and diluted in the seawater delivered by oceanwaves, it is required to provision that when the floating platform 610gradually rises (see FIG. 6D) and magnesium hydroxide gets dilutes up tothe point when the opening 630 gets completely filled in with seawaterwith no solid magnesium hydroxide available (see FIG. 6E), the uppersurface 632 of the opening 630 in FIG. 6E is still equal or below theheight H of the ocean waves in the part of the ocean where the floatingplatform 610 is placed.

A wireless weight sensor 660 may be placed on the floating platform 600or nearby to measure and report on the amount of Mg(OH)₂ remaining, andanother sensor or sensors 670 measuring the characteristics of thesurrounding seawater including pH pCO₂ and conductivity.

If pH or other parameter near the platform reaches a predetermined upperthreshold, for example pH of about 9.4-10, the sensor 670 sends a signalto raise the platform so as to reduce the submerge surface beingcontacted by the seawater and thus the generation of the dissolvedMg(OH)₂. The platform may be raised (or lower) for example by pneumaticmeans wherein air is pumped into (or released) from the buoyancy devices(612) thus affecting platform height and the volume of seawatercontacting the solid Mg(OH)2 surfaces.

The opening or hull 630 may also have a permeable side opening 650 orbottom opening 661 having a membrane, diaphragm or semi permeablebarrier, the permeable section 650 positioned at least partially belowthe waterline of the floating platform 600 in order to allow the inflowand/or outflow of seawater to effect Mg(OH)₂ dissolution and dischargerate, when there are no waves of sufficient magnitude in the ocean, orin addition to in addition to the dissolution of Mg(OH)₂ in the hull 630by ocean waves delivering the seawater at the top of the hull 630.

Preferably, the permeability of the membrane, diaphragm, semi permeablebarrier or cloth is chosen to allow the passage of Mg²⁺ and OH⁻ ionsinto the surrounding seawater at a rate not to exceeding about 3mmoles/(L×m²×day). Full saturation of the seawater with metal hydroxideis not required since at least some CO₂ reaction with dissolved metalhydroxide will occur at any concentration above ambient seawater levels.

The permeability of the barrier is chosen to maximize flow rate ofseawater across the membrane, driven by wave action, tidal forces, oceancurrents or mechanical means, while limiting the escape of solidparticles.

The choice of membrane is therefore chosen as the largest pore size thatis still smaller than the smallest hydroxide particle size. Due to thisrestriction, it is advantageous to supply the floating platform 600 withmetal hydroxide solids that are significantly larger than the pore sizeof the membrane. For example, supplying the floating platform withnatural brucite aggregate whose mean diameter per individual aggregateis 10 cm can be retained by a semi-permeable barrier with a mean poresize of 10 microns so as to retain all but the smallest particlesproduced via dissolution and fragmentation of the aggregate whileallowing sufficient water in and out of the platform.

In operation of the floating platform 600, the seawater thus contactingthe Mg(OH)₂ mass in the hull 630 is allowed to become partially orcompletely saturated with dissolved Mg(OH)₂. The Mg(OH)₂-enrichedseawater is then continuously released into the ocean.

FIG. 7A schematically illustrates a floating platform 700 of anotherembodiment of the invention for distribution of dissolved magnesiumhydroxide in the ocean. The floating platform 700 of FIG. 7A has a hullor opening 730, which may be either similar to the opening of 630 ofFIG. 6A, or alternatively, the hull 730 may have a form of a closedcavity, with no exposure to ocean waves. FIG. 7A shows the hull 730 inthe form of the closed cavity.

The floating platform 700 has a container 732 which is illustrated inmore detail in FIG. 7B. The container 732 has solid magnesium hydroxidestored in an upper portion 720 of the container 732, and a dispenser 734in the lower portion of the container 732 for dispensing a predeterminedor required amount of magnesium hydroxide from the container 732 in thehull 730, the required amount being calculated by the computer 740. Thedispenser 734 is operable in response to a dispensing signal, which istriggered by a sensor 736 associated with the dispenser 734. Uponreceiving the dispensing signal, and assuming the dispenser 734 isalready loaded with the predetermined amount of magnesium hydroxide withan upper door 738 of the dispenser 734 being closed, a lower door 739 ofthe dispenser is opened, and the predetermined amount of magnesiumhydroxide is released in the hull 730. Next, the lower door 739 of thedispenser 734 is closed, the upper door 738 is opened, allowing loadingthe next predetermined amount of magnesium hydroxide in the dispenser734, and the the upper door 738 is closed again, thus preparing thedispenser 734 for the next operation.

In a simple form, the dispensing sensor 736 may be implemented, forexample, as a timer, which triggers the control signal upon elapsing acertain predetermined period of time since the previous operation of thedispenser 734, for example 12 hours or one day. Alternatively, thecontrol signal may be generated by a computer 740 having a processor anda memory, controlling the entire operation of the floating platform 700.

The container 732 has a weight sensor 747 for measuring and reporting onthe amount of Mg(OH)₂ remaining in the upper portion 720 of thecontainer to the computer 740. The computer 740 controls a communicationsensor 741 for sending a request to.

The floating platform also has a communication sensor 741 controlled bythe computer 740 for sending a communication signal to outside entities,for example a request to a ship or a ground control station within anoperational range of the communication sensor 741, for loading magnesiumhydroxide in the container 732 or relocating the floating platform 700to another location in the ocean.

The floating platform 700 also has a first pump 744 for pumping seawaterfrom the ocean into the hull 730 for diluting the magnesium hydroxide,and a second pump 746 for pumping the solution of the magnesiumhydroxide dissolved in the seawater out of the hull 730 to the ocean.Both the first pump 744 and the second pump 746 are controlled by theprocessor 740.

Further, the floating platform 700 has an internal sensor 750, forexample a pH sensor, for measuring pH of the solution in the hull 730,and an external sensor 752 for measuring characteristics of the seawatersurrounding the floating platform 700, for example, pH and/or inorganiccarbon of the surrounding seawater in the vicinity of the floatingplatform 700, for example within 100-300 meters. Assuming pH of thesolution, prepared in the hull 730, is already within requiredenvironmental limits, for example in the range of about 9.0-9.4, then pHsensor 752 may become optional.

Additionally, the floating platform has a sensor 733 for measuring alower water level inside the hull 730 and another sensor 735 formeasuring an upper water level inside the hull 730 to make sure thewater level in the hull 730 is within a predetermined range.

The floating platform 700 of FIG. 7A also has an anchor 680 similar tothat of FIG. 6A for anchoring the floating platform 700 in a requiredarea in the ocean. The hull 730 may optionally have a side opening (notshown) similar to the opening 650 of FIG. 6A, or a bottom opening (notshown) similar to the bottom opening 661 of FIG. 6A, to be used when thefirst and second pumps 744 and 746 are not working, and the platform 700is actually converted into a passive floating platform 600 of FIG. 6A.

The floating platform 700 has two modes if operation.

FIG. 8A shows a flow-chart 800 illustrating a first, intermittent modeof operation of the floating platform 700. Upon start (box 802), apredetermined volume of seawater is pumped into the hull 730 using thefirst pump 744 (box 804). Then a predetermined amount of magnesiumhydroxide is added to the seawater in hull 730 (box 806), followed bymixing the solution or waiting a predetermined period of time untilsubstantially all magnesium hydroxide is diluted in the hull 730 (box808) to reach a pH of the solution within a predetermined pH range, forexample <9.

As soon as magnesium hydroxide is substantially dissolved, activatingthe second pump 746 and pumping the solution with magnesium hydroxideout of the hull 730 into the seawater surrounding the floating platform(box 810). Wait for a predetermined time interval, for example one day,(box 812), and check if there is sufficient amount of magnesiumhydroxide in the container 732 (box 814). If yes (exit Yes from box814), repeat the steps 804-812 all over again. If no (exit NO from box814), terminate the operation of the floating platform 700 until a newload of solid magnesium hydroxide is loaded in the container 732.

Please note that sensors 750, 752, 733 and 735 may be optional for theintermittent mode of operation of the floating platform 700, and thus,the floating platform 700 may be somewhat simplified.

In a modification of the above embodiment of FIG. 7A, the first pump 744and the second pump 746 may be replaced with a single bidirectional pumpcontrolled by the computer 740 and capable of performing bothoperations: pumping the seawater into the hull 730 and pumping thealkaline solution out of the hull 730 into the ocean, eithersimultaneously in both directions or intermittently in each direction.

FIG. 8B shows a flow-chart 900 illustrating a second, continuous mode ofoperation of the floating platform 700.

Upon Start (box 902) the procedure 900 checks if there is sufficientamount of magnesium hydroxide in the container 732 of the floatingplatform 700 (box 904). If no (exit No from box 904), the proceduresends a request to external sources to load the container 732 with a newload of magnesium hydroxide and waits (box 906), periodically checkingif the container 732 has been loaded (loop from the box 906 to box 902and box 904). If yes (exit Yes from box 904), the computer 740 instructsthe first pump 744 to pump seawater into the hull 730 (box 908),followed by checking if the water level in the hull 730 is within apredetermined range (box 910). If no (exit No from box 910), theprocedure returns back to the step 908. If yes (exit Yes from box 910),the computer instructs the dispenser 734 to dispense magnesium hydroxideinto the hull 730 (box 912), followed by checking if a pH level in theseawater in the hull 730 is within a predetermined pH range (box 914)not to exceed a permitted environmental pH threshold, for example pHfrom about 9.0 to about 9.4. If no (exit No from box 914), the procedurereturns back to box 912. If yes (exit yes from box 914), the computerinstructs the second pump 746 to pump the Mg(OH)₂ solution out of thehull 730 into the ocean (box 916), followed further checking if thewater level in the hull 730 is within a predetermined range (box 922).If yes (exit Yes from box 922), the procedure returns back to box 916.If no (exit No from box 922), the procedure returns back to box 904.

Thus, the embodiments of FIGS. 8A and 8B allow the Mg(OH)₂-enrichedseawater to be periodically or continuously released into the ocean.

As mentioned above, the surface area of the solid Mg(OH)₂ relative tothe flushing rate of the seawater in contact with the solid Mg(OH)₂ arecontrolled such that a dissolution rate of not more than than 3mmoles/(L×m²×day) (0.175 g Mg(OH)₂/(L×m²×day) is maintained. Forexample, if 10,000 L of water is allowed in and out of the platform perday to contact the solid Mg(OH)₂, this limits the maximum totaldischarge of dissolved Mg(OH)₂ to 1,750 g/day that in turn limits themaximum size of the solid Mg(OH)₂ surface area exposed to the contactingseawater to 10,000 m².

Such surface area can be provided by a cube of 41 meters per side or asphere with a maximum diameter of 56.4 meters. Since solid Mg(OH)₂objects this size may be difficult to manufacture and handle, it followsthat a multitude of much smaller objects that together maximally presentthe 10,000 m² required may be desirable. For example 10,000 cubes eachpresenting 1 m² (0.41 m on a side) or 100,000 spheres each presenting0.1 m² (0.178 m in diameter). The shapes of the solid objects may beirregular as long as 10,000 m² of solid Mg(OH)₂ surface area ispresented. The solid forms may be manufactured from synthetic Mg(OH)₂ orfrom natural Mg(OH)2 (brucite mineral), or may be used in the forms andsizes naturally resulting from the Mg(OH)₂ synthesis or mineralextraction process without further shaping or sizing. Furthermore, thedaily amount of water contacting the mass or masses can be varied toaccommodate specific mass surface areas presented.

It is understood that as such solid masses dissolve their mass andsurface area decreases and hence the production rate of dissolvedMg(OH)₂ to seawater declines. This can be countered by periodicallyadding additional Mg(OH)₂ mass (surface area) to the floating platform600 or 700 as described above. The volume/day of contacting seawater canalso be adjusted to maintain a specific Mg(OH)₂ concentration in thewater discharged to the ocean.

Powdered Mg(OH)₂ will dissolve and saturate in seawater yielding a pH of9.4 within 1 hour. In order to maintain a maximum pH of 9 in the bulk ofthe surrounding seawater, a constant flow of seawater through the hull410 is required. This flow rate is set based on the surface area of theMg(OH)₂ exposed to the seawater in order to achieve a maximum residencetime of about 1 hour. Saturated seawater is then discharged into an areawith a corresponding refresh rate to maintain a maximum pH of 9 due todilution effects. This discharge is achieved through natural wave, tidalor current action or through pumping, as described above.

Once released, dilution of the added metal hydroxide to seawater canquickly reduce undesirable chemical conditions as distance and time fromrelease increase. For example, depending on ocean conditions, dilutionby a factor of 100 can occur in the ocean 10 minutes after discharge. Ifthat discharge has a pH of 9, in 10 minutes the mixing of 1 partdischarge water with 100 parts ambient seawater with a pH of 8 wouldresult a mixed seawater pH of 8.004. Thus, the interplay betweendischarge release rate and dilution with seawater determines the arealextent and duration of undesirable seawater chemistry if present.

The preceding methods then control the rate at which dissolved Mg(OH)₂is produced and released into the surrounding seawater, the degree andrate of seawater dissolved Mg(OH)₂ dilution, and thus the magnitude ofpH increase and associated chemical changes at any given place and timein the surrounding seawater.

The floating platform 600 may be placed and anchored at sea, asschematically illustrated by the anchor 680 in FIG. 6A or FIG. 7A. Theanchoring 680 could be either provided by a direct connection to theseafloor or through anchoring systems that allow a certain amount ofdrift. Once set in place, seawater, through the action of wind orcurrents, flows through the permeable material and over and throughchannels in the Mg(OH)₂. This erodes the hydroxide, raising the pH ofthe surrounding seawater.

Conveniently, the floating platform 600 may be anchored, for example inan area of high flow such as a tidal area, an estuary or other rivermouth to more rapidly weather and dilute the hydroxide.

Alternatively, the floating platform can be allowed to passively drifton the ocean surface thus adding dissolved Mg(OH)₂ along a path dictatedby surface ocean currents and winds. Furthermore, the platform can beoutfitted with facilities for autonomous navigation and propulsion so asto allow the platform to stay in one spot or to traverse a prescribedroute on the ocean surface, for example to eventually return to astation that would provide resupply of solid Mg(OH)₂ and allowmaintenance of the platform.

Also, the floating platform 600 or 700 may be pulled behind the ship, asshown in FIG. 9 .

In yet another embodiment, illustrated in FIG. 10 , the floatingplatform 600 is fitted with a device 680 such as an air pump 680 thatfacilitates the contacting of air with the seawater within the hull 630that contains dissolved Mg(OH)₂. By bubbling air through the seawaterenriched with dissolved Mg(OH)₂ the transfer of CO₂ from air into theseawater is accelerated, thus speeding up CO₂ removal and sequestrationfrom air, raising seawater DIC, and beneficially lowering the seawater'spH from otherwise more elevated values.

In another embodiment illustrates in FIG. 11 , the platform 1100containing the Mg(OH)₂ does not float, but is secured at the ocean'sshoreline or within water discharging into the ocean such as a river,stream, wastewater discharge or other natural or artificial discharge ofwater to the ocean. Thus, the flow of water provided by ocean waves,currents or water flowing 1140 through and around a mass of solidMg(OH)₂ 620 encased in a porous container 1110 and is at least partiallysubmerged below the water level 1130 causes the dissolution of Mg(OH)₂into water that ultimately discharges 1150 into the ocean. The Mg(OH)₂may be contained in porous vessels or containers that allow thecontacting of water with the solid Mg(OH)₂ to effect Mg(OH)₂ dissolutionand the production of dissolved Mg(OH)₂ that immediately or eventuallyenters the ocean. Such containers or vessel include but are not limitedto burlap bags, screened metal containers, or troughs containingsynthetic or natural Mg(OH)₂ solids around or through which seawater orwater (ultimately discharging to the ocean) passes.

The geographic scale of the present invention and its global capacity toremove and sequester atmospheric CO₂ may be increased through thedeployment of multiple platforms 600, 700 or 1100.

By coloring the above-ocean surfaces of the floating platforms white ora light color the platforms could service to reduce surface oceanalbedo. This would provide a means of reducing the amount of solarenergy reaching the ocean and thus beneficially reduce surface oceanwarming that is otherwise occurring as a consequence of elevatedanthropogenic CO₂ in the atmosphere.

In yet one more embodiment, the floating platform 600 or 700 is deployedto an area of ocean upwelling where surface seawater is supersaturatedin CO₂ relative to the overlying atmosphere and where the addition ofdissolved magnesium hydroxide beneficially captures and sequestersdissolved CO₂ that would otherwise escape to the atmosphere.

While the use of Mg(OH)₂ is discussed in the above embodiments, it isunderstood that other metal hydroxides or other sparingly or fullysoluble alkaline materials may be similarly be used in the invention.Here, the specific rates of alkaline material dissolution in seawaterare again used to design platforms that allow the release of dissolvedmetal hydroxide to the surface ocean such that seawater pH does notexceed 9 and/or outward chemical and biological effects are avoided.Such alkaline materials include but are not limited to: Ca(OH)₂, NaOH,KOH, MgO, CaO, CaSiO₄ and Mg₂SiO₄.

Although specific embodiments of the invention have been described indetail, it should be understood that the described embodiments areintended to be illustrative and not restrictive. Various changes andmodifications of the embodiments shown in the drawings and described inthe specification may be made within the scope of the following claimswithout departing from the scope of the invention in its broader aspect.

1. A floating platform for sequestering carbon dioxide using a body ofwater, comprising: (a) a vessel for holding solid metal hydroxide; (b)means for exposing the solid metal hydroxide to a flow of water tocreate a solution of a metal hydroxide having a pH level; (b) means forreleasing the solution containing the metal hydroxide into the body ofwater, including causing a reaction of the released metal hydroxide withthe carbon dioxide present in the body of water, thereby producing oneor more of a metal carbonate and metal bicarbonate below a saturationconcentration in the body of water, thus sequestering the carbon dioxideusing a body of water; and (c) means for choosing a metal in the metalhydroxide, a rate of the releasing the solution containing the metalhydroxide into the body of water, and a flow rate of the flow of waterso that to substantially maintain the solution containing the metalhydroxide at said pH level that is defined as environmentally safe. 2.The floating platform of claim 1, wherein the metal hydroxide ismagnesium hydroxide.
 3. The floating platform of claim 2, wherein therate of the releasing is about 3 mmoles dissolved Mg(OH)₂/(L×m²×day). 4.The floating platform of claim 1, wherein the pH level is from about 9.0to about 9.4.
 5. The floating platform of claim 1, wherein the pH levelis not exceeding 9.4.
 6. A method for sequestering carbon dioxide usinga body of water, comprising: (a) in a vessel containing solid metalhydroxide, exposing the solid metal hydroxide to a flow of water tocreate a solution containing the metal hydroxide and having a pH level;(b) releasing the solution containing the metal hydroxide into the bodyof water, including causing a reaction of the released metal hydroxidewith the carbon dioxide present in the body of water, thereby producingone or more of a metal carbonate and metal bicarbonate at belowsaturation concentration in the body of water, thus sequestering thecarbon dioxide using a body of water; (c) choosing a rate of thereleasing the solution containing the metal hydroxide into the body ofwater in the step (b) and a flow rate of the flow of water in the step(a) so that to substantially maintain the solution containing the metalhydroxide at at said pH level that is defined as environmentally safe.7. The method of claim 6, wherein the metal hydroxide is magnesiumhydroxide.
 8. The floating platform of claim 7, wherein the step (c)comprises choosing the rate of the releasing equal to about 3 mmolesdissolved Mg(OH)₂/(L×m²×day).
 9. The method of claim 6, wherein the pHlevel is from about 9.0 to about 9.4.
 10. The method of claim 9, whereinthe pH level is not exceeding 9.4.